using System;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Security;

namespace Org.BouncyCastle.Crypto.Paddings
{

	/// <summary> A padder that adds Trailing-Bit-Compliment padding to a block.
	/// <p>
	/// This padding pads the block out compliment of the last bit
	/// of the plain text.
	/// </p>
	/// </summary>
	public class TbcPadding
		: IBlockCipherPadding
	{
		/// <summary> Return the name of the algorithm the cipher implements.</summary>
		/// <returns> the name of the algorithm the cipher implements.
		/// </returns>
		public string PaddingName
		{
			get { return "TBC"; }
		}

		/// <summary> Initialise the padder.</summary>
		/// <param name="random">- a SecureRandom if available.
		/// </param>
		public virtual void Init(SecureRandom random)
		{
			// nothing to do.
		}

		/// <summary> add the pad bytes to the passed in block, returning the
		/// number of bytes added.
		/// <p>
		/// Note: this assumes that the last block of plain text is always
		/// passed to it inside in. i.e. if inOff is zero, indicating the
		/// entire block is to be overwritten with padding the value of in
		/// should be the same as the last block of plain text.
		/// </p>
		/// </summary>
		public virtual int AddPadding(byte[] input, int inOff)
		{
			int count = input.Length - inOff;
			byte code;

			if (inOff > 0)
			{
				code = (byte)((input[inOff - 1] & 0x01) == 0?0xff:0x00);
			}
			else
			{
				code = (byte)((input[input.Length - 1] & 0x01) == 0?0xff:0x00);
			}

			while (inOff < input.Length)
			{
				input[inOff] = code;
				inOff++;
			}

			return count;
		}

		/// <summary> return the number of pad bytes present in the block.</summary>
		public virtual int PadCount(byte[] input)
		{
			byte code = input[input.Length - 1];

			int index = input.Length - 1;
			while (index > 0 && input[index - 1] == code)
			{
				index--;
			}

			return input.Length - index;
		}
	}
}
